Lock-in thermography (LIT) is a known method for examining internal structures of objects by applying periodic energy waves and analyzing the resulting temperature profile of the area to be examined. Generally, when the input wave penetrates an object's surface, the wave is partially reflected in regions where the tissue to be examined is non-homogenous. The reflected wave interferes with the input wave and creates interference patterns in the local surface temperature of the object. Analysis of these interference patterns makes it possible to identify the internal structure of said object. LIT may e.g. be used for characterizing tissue in-vivo and detect skin lesions like for example regions with cancerous cells, such that identification of such tumors or malign cells, e.g. in case of skin cancer, can be performed without having to extract samples from the tissue.
With respect to this type of application it has been discovered that usage of nanoparticles in detection, characterization and potentially destruction of cancerous tumors is a very promising path in this field, particularly in view of the potential possibility of destroying tumors by non-invasive techniques.
Nanoparticles, i.e. materials at nanoscale with typical sizes of 1 to 100 nm in at least one dimension, offer a variety of unique properties. The capability of producing heat when exposed to an alternating magnetic field (AMF) is an extraordinary property of magnetic nanoparticles (MNPs). Due to the thermal energy, which is produced by MNPs when they are exposed to an AMF, they can be used for cancer therapy in a process generally known as magnetic hyperthermia. Efficient heating depends on different variables including magnetic field strength or frequency and chemistry, size and surface of the used MNPs. In order to choose the most efficient MNPs to be used at lowest possible concentration for magnetic hyperthermia treatment, this heating efficiency must be determined. The synthesis of typically used MNPs (mostly superparamagnetic iron oxide nanoparticles) is well described in the literature and different sizes and surfaces of these particles are commercially available (e.g. Chemicell, Advanced Magnetics, Nanocomposix).
The most commonly used factor to quantitatively reflect the heating power of iron oxide nanoparticles is the Specific Absorption Rate (SAR) in W·g−1, which is defined as the rate at which electromagnetic energy from a defined amount of magnetic material is absorbed by a unit mass of a material
  SAR  =      P          m      NP      where P is the heat power dissipated by the magnetic nanoparticles and mNP is the total mass of magnetic material. However, it must be noted that the SAR-value does not cover all variables; as mentioned above, the heating power is also depending on external factors like the magnetic field strength and frequency, making a further normalization step necessary. In this context, it has been shown that, at low frequencies and field strengths (i.e. which are mostly used in clinical settings), the SAR can be considered as a function of the frequency and square of the magnetic field. Consequently, the SAR can be further normalized in accordance with these assumptions. This value is referred to as the Intrinsic Loss of Power (ILP) in nH·m2·kg−1 a system-independent parameter which is defined as:
  ILP  =      SAR                  H        2            ⁢      f      
where H is the strength of the AMF and f its frequency. The ILP is particularly useful in comparing results obtained by different experimental settings, batches and research groups. However, it should be kept in mind that this value represents only an approximation and only applies to a narrow experimental window.
The publication “Accuracy of available methods for quantifying the heat power generation of nanoparticles for magnetic hyperthermia” by I. Andreu and E. Natividad, Int. J. Hyperthermia, 29:8, 739-751 reviews currently available methods of SAR-determination of MNPs. In the paper, the conclusion is drawn that the determination of the heat efficiency of MNPs via thermal imaging requires a set up providing quasi-adiabatic conditions, i.e. the measured sample has to be thermally insulated in combination with the measurement conditions.
Standard calorimetric methods (e.g. fiberoptic cables or thermocouples) are the currently used approaches to evaluate the heating capabilities. However, they only provide measurement data from single one-dimensional points, and are consequently very limited in regard to reproducibility (in regard to sensor positioning), accuracy and precision. Although the set-up is easy to install, the correct fitting of the obtained data is challenging and the measurements are time consuming and invasive. Additionally, due to convection or conduction, the heat loss during measurements is not taken into account.
Beside the already mentioned applications, LIT is currently a standard method used for testing composite materials and electronic components (e.g. solar panels) and is based on the modulation of the thermal radiation. In short, the heat-generating stimulus (in this case, the AMF) is periodically modulated over a specified amount of cycles while an infrared camera continuously records the thermal events. The data acquisition and output differ from the other methods in that rather than recording variations of temperature over time the amplitude or the magnitude of the temperature oscillations during the modulation cycles is determined.
Generally, usage of nanoparticles in LIT gives rise to problems involving a number of factors like experimental set-ups and measurement conditions. Furthermore, commercially available MNPs show non-negligible batch to batch variations with respect to magnetic behavior, size distribution, crystallinity or surface.
Thus, due to the variations of the physical MNP-properties resulting from the manufacturing process, a pre-characterization of MNPs in terms of measuring heat production of magnetic nanoparticles in an alternating magnetic field before their usage in the actual application is very important for achieving good results with respect to the specific application.